In internal combustion engines for motor vehicles, there are large variations in the running velocity of the vehicle, namely in the rotating speed of the engine and the load, and high performance characteristics such as low fuel consumption, production of a minimal quantity of harmful exhaust gases and the like are required in various kinds of operating conditions in combination with both of these variation factors. For this purpose, the air-fuel ratio must be properly controlled in accordance with various kinds of operating conditions of the engine.
As a method of properly controlling the air-fuel ratio, there has been used an air-fuel ratio control apparatus of the feedback type in which an exhaust gas sensor is provided to detect the concentration of a specific component in the exhaust gases, for example an O.sub.2 sensor to detect the concentration of oxygen, and a control valve is provided to control the supply quantity of bleed air and is made operative in response to an output signal from the O.sub.2 sensor to thereby adjust the air-fuel ratio. In this way, the air-fuel ratio is properly controlled so as to always obtain the best combustion condition in accordance with the various kinds of operating states.
With this control apparatus, however, in the transient operating state in which the engine operating condition is shifted from the idling operating state to the partial or acceleration operating state or to the operating state at a high altitude, then as shown in FIGS. 1 and 2, the correcting operation width H of the air-fuel ratio required in operating conditions at a high altitude is larger than the correcting operation width h needed for normal operating conditions at a standard low altitude, due to the decrease in the air concentration and the like. This causes the control of the air-fuel ratio by way of a correction signal to be delayed after an increase in engine speed, so that there is the drawback that the air-fuel ratio is enriched and the quantity of CO as a harmful exhaust component is increased, for example by an amount indicated by the hatched region in FIG. 4D.
On the other hand, in conventional air-fuel ratio control devices for internal combustion engines, an output signal from an exhaust sensor, for example an O.sub.2 sensor, is inputted to the electronic control unit (ECU), and the valve provided in the carburetor is controlled by this ECU in a feedback manner, thereby controlling the air-fuel ratio.
However, in such conventional control devices, as mentioned above, the correcting operation width in the operating condition of the engine at a high altitude becomes larger than that in the normal operating condition at a standard low altitude in the transient operating condition in which the engine operating state is shifted from the idling state to the partial operating state, as shown in FIGS. 1 and 2.
Consequently, the correcting of the air-fuel ratio is delayed, causing the drawback that the quantity of CO produced as a harmful exhaust gas is increased as a result of the enriched air-fuel ratio, as indicated by the hatched region in FIG. 6D.